Intermittent Fasting And Athletic Performance

By Lindsey Esposito, MS, RD, CSSD

Collegiate and Professional Sports Dietitians Association (CPSDA) Member

Fad diets have taken over the nutrition industry, many of which promise quick weight loss or set guidelines that are not sustainable. One that has become increasingly more popular because of recommendations from doctors or endorsements by celebrities and athletes is intermittent fasting. Fasting is described as a voluntary or involuntary abstention from food, with food restriction being either partial or total. Water consumption may or may not be included depending on the type of fasting. The premise of intermittent fasting is to curve hunger, resulting in a restriction of calories due to a shorter feeding window. Some early studies have found promising results, including behaviors associated with enhanced weight loss and improved body composition and metabolic markers. Although research on intermittent fasting involving the athletic population remains limited, athletes continue to look at this diet as a potential way to gain a competitive advantage over their opponents.

There are several different variations of fasting, each with their own specific set of rules and guidelines. Alternate day fasting is where food is restricted for a day, followed by a day when you eat ad libitum. Based on data involving animals and the general population, this fasting style may result in a modest caloric restriction, however, it has not been determined as being more effective than other calorie restriction methods at this time. The next type of fasting variation includes modified restricting regimens, such as the 5:2 diet. On the 5:2 diet, the individual engages in a normal eating pattern for five days, followed by two days of fasting. On the two fasting days, calorie intake is typically reduced to 25% or less of the individual’s daily needs. Again, while weight loss has been reported as a benefit to this fasting style, there is little evidence on this method being more effective than other restriction methods. Time-restrictive feeding is where fasting intervals range anywhere from 12-20 hours per day, with the most common being the 16/8 method (fasting for 16 hours, eating for 8). This type of fasting is similar to Ramadan fasting without the sunrise/sunset guidelines and the restriction of fluids. Lastly, the Warrior Diet or OMAD is a modified version of time-restricted feeding, where you eat very little during the day and feast at night.

Common Intermittent Fasting Methods
Alternate Day Fasting Restrict food for one day, eat ad libitum the following day. Some individuals do a full 24-hour fast while others eat 500-600 calories on fasting days.
5:2 Diet Eat normally for five days, fast for two days. Fasting days may be consecutive or separated. During fast days, intake is typically reduced to ≤25% of daily calories, or 500-600 calories total.
16/8 Method Fast for 16 hours, eat for 8 hours. Most individuals finish their evening meal around 8 PM, miss breakfast, and do not eat again until 12 PM. Other time restricted feeding methods can have fasting intervals between 12 and 20 hours.
Warrior Diet Eat very little during a 20-hour fasting period (ex: raw fruits and veggies), then eat one large meal at night. Fasting window is typically 4 hours.
Eat-Stop-Eat Eating no food for a full 24 hours, 1-2 times per week.


Focusing on the athletic population, the big question to assess is whether intermittent fasting promotes improved sports performance or not. It is difficult to draw conclusions regarding intermittent fasting and its influence on sports performance, as research involving elite athletes is essentially non-existent. In theory, intermittent low-energy availability should trigger adaptive responses such as mitochondrial biogenesis and increased fat oxidation, thus improving performance (especially in endurance athletes); however, research has not yet made these correlations. Most of the studies involving fasting and athletes include participants who observe Ramadan, with only a few assessing fasting outside of this religious observation. When looking at recent endurance, high-intensity, and resistance-based exercise studies, results are variable but all largely show there is no benefit to athletic performance while fasting.

High-intensity exercise is dependent on the availability of carbohydrates, therefore there is a concern about how intermittent fasting affects high-intensity exercise performance. Many recent studies have analyzed repeated sprints during fasting while observing Ramadan and have found negative effects. Even when Aziz et al. (2017) controlled for pre-exercise conditions such as sleep, nutrition, and training loads, sprint times during a 60-min intermittent sprint exercise were significantly reduced. Non-Ramadan studies that involved cycling and rowing time trials have also cited performance decrements while fasting (Levy & Chu, 2019). Naharudin & Yusof (2018) looked at the impact of intermittent fasting (omitting lunch, ~40% of total daily calories) on Wingate anaerobic power and time to exhaustion while cycling. This study was interesting because while peak power output and time to exhaustion were reduced in the beginning of the 10-day study period, the effects of intermittent fasting were restored after just a few days of fasting. This short-term fast spanning ten days caused deteriorated high-intensity performance, however, results suggest that the body may adapt and allow for recovery of performance over a longer fasting period (Naharudin & Yusof, 2018).

In regards to endurance exercise, most studies have shown either variable negative effects or no effect at all, while no studies have shown any true benefits to performance (Levy & Chu, 2019). For example, Clayton et al. (2015) reported that cyclists who skipped breakfast performed at a lower workload during a 30-minute performance test later in the evening. Excluding Ramadan studies, Aird et al. (2018) completed a systemic review and meta-analysis on fasting and aerobic performance. Outcomes included 57% and 46% of studies showing no difference between the fasting and fed groups for exercise lasting <60 minutes and >60 minutes respectively. Years prior, Zerguini et al. (2007) demonstrated that speed, agility, dribbling speed, and endurance of professional soccer players were reduced during Ramadan. One study conducted in mice showed improved endurance performance while eating a diet pattern that resembled the 5:2 diet; however, these results have yet to be replicated in human studies (Levy & Chu, 2019).

Most results from resistance training studies show that intermittent fasting causes minimal negative effects. Karli et al. (2007) found no negative effects on power output in Turkish male power athletes who were fasting as long as they maintained their daily energy needs and hours of sleep. In the study conducted by Moro et al. (2016), anabolic hormones such as testosterone and IGF-1 were reduced in the resistance-trained males participating in time-restricted feeding (16/8 method) while there were no significant changes in the participants consuming a normal diet. This may lead to a negative effect on performance in the long run. . More recently, a review study revealed that carbohydrate restriction, often caused by intermittent fasting, reduces muscle hypertrophy and the athlete’s ability to undertake lengthy training sessions (Cholewa, Newmire, & Zanchi, 2019). Indirectly related to the physical aspects of sports performance, recent findings suggest that intermittent fasting might affect the relationship between cytokine expression in the brain and cognitive deficits, including memory deficits (Cherif, Roelands, Meeusen, & Chamari, 2015). Certain mental health qualities such as coping and decision-making strategies can also be negatively influenced by daylight fasting, thus affecting an athlete’s level of performance (Cherif et al., 2015).

As previously mentioned, it is difficult to apply the findings from Ramadan-based studies to form conclusions about intermittent fasting. While time-restricted feeding does not appear to affect sleeping patterns, studies showed differently in those observing Ramadan. Ramadan participants tend to consume less calories than normal and often experience disturbances in their circadian rhythm due to overnight eating. Roky et al. (2001) discovered a delay in sleep onset and an increase in nocturnal body temperature during Ramdan, which was potentially attributed to the participants’ inverted meal and drinking schedule. This displacement of a normal circadian pattern of eating and drinking can affect an athlete’s total calorie intake, in addition to their hydration status. Overall, sleep disturbances and depletion of glycogen and fluid reserves over a long day of fasting will likely cause a negative impact on performance.

A recent review published in 2012 mentioned that as long as athletes maintain their total intake of calories and micronutrients, in addition to their typical sleep quality, the negative effects of fasting on performance are less likely (Chaouachi, Leiper, Chtourou, Aziz, & Karim, 2012). The problem is that many athletes are not getting enough sleep and do not meet their daily calorie needs as it is. In fact, in a recent study of 29 varsity teams (628 collegiate athletes), 51% reported high levels of excessive daytime sleepiness based on the Epworth Sleepiness Scale and 42% experienced poor sleep quality as measured by the Pittsburgh Sleep Quality Index (Mah, Kezirian, Marcello, & Dement, 2018). Additionally, only 25% of collegiate athletes reported at least eight hours of sleep per night in the NCAA GOALS survey (National Collegiate Athletic Association, 2019). When it comes to meeting daily calorie needs, Shriver et al. (2013) looked at three-day food logs from 52 NCAA Division I female athletes. Total energy and carbohydrate intakes were below sports nutrition recommendations, with only 9% of the athletes meeting their energy needs (Shriver, Betts, & Wollenberg, 2013). If athletes are not meeting their dietary needs without a restricted feeding window, it will be extremely difficult for them to meet their high rates of energy expenditure while participating in intermittent fasting. For example, fitting in 4,500 calories in a small number of meals or a shortened feeding window would be difficult to accomplish, especially because athletes may start to feel full quickly. When an athlete’s overall calorie and carbohydrate needs are not being met, a small contribution of energy may come from plasma protein and amino acids, potentially causing a reduction in muscle and concomitant strength and performance.

Not only can a restricted feeding window cause inadequate muscle recovery, but it can also cause a higher risk of syncope for athletes who have early morning practice, as most fasting days/times do not include breakfast. If an athlete is participating in the 16/8 fasting method and has training in the morning, his or her last meal should be high in carbohydrates to ensure glycogen stores have been filled. If the athlete trains earlier in the day and cannot eat until after his or her training session, training adaptations and performance may be hindered; however, if the athlete trains later in the day, sufficient energy and nutrients may be able to be provided in the pre- and post-exercise meals to support his or her training. While total energy and nutrient intake continues to be stressed more than the timing of when it comes, multiple pieces of evidence support the idea that consuming 20-40 grams of protein (0.25-0.40 g/kg body mass) every three to four hours more favorably increases muscle protein synthesis, which in turn affects muscle growth and recovery (Kerksick et al., 2017). Intermittent fasting limits the athletes’ ability to spread out their protein intake and although the impact may be small, it could ultimately make a huge difference for elite athletes in the end.

Few studies have looked at hydration status during fasting. During this religious observation, the absence of fluid intake during daylight hours has led to a progressive loss of body water over the course of the day that is typically replaced each night (Maughan & Shirreffs, 2012). Most researchers agree that as little as a 1-2% decrease in body mass from water can impair both physical and cognitive performance. Looking at elite athletes’ training regimens, many of them are susceptible to having larger body water losses through sweat. Larger body water losses diminish performance even further and typically occur when exercise takes place in hot weather, when the duration is longer than an hour, and when training or competition sessions take place in a single day (Maughan & Shirreffs, 2012). Recovery and performance is impaired by the absence of fluid intake, so it is important for athletes participating in Ramadan to maximize nighttime and early morning (before sunrise) fluid and carbohydrate intake. If engaging in non-religious fasting, fluid consumption should be encouraged during the fasting hours.

One benefit to intermittent fasting is a potential decrease in body weight and or body fat percentage. To lose fat mass, the athlete ultimately has to be in a calorie deficit, meaning he or she is consuming less calories than they are burning. If gaining muscle mass is the athlete’s goal, it is imperative that he or she consume enough total calories and protein throughout the day. Moro et al. (2016) assessed the effects of the 16/8 method in males undergoing a standardized resistance training program compared to controls, where both groups met 100% of their determined energy needs. After eight weeks, a significant decrease in fat mass and a reduction in inflammatory markers were observed in the time-restricted feeding group in comparison to the control group (Moro et al., 2016). Fat-free mass, maximal strength, and the muscle area of both the thigh and arm were maintained in both groups (Moro et al., 2016). In a study looking at resistance-trained females, similar gains in fat free mass, muscle hypertrophy, and muscular performance were achieved in the control group and the time-restricted feeding group (Tinsley et al., 2019). Energy and protein intake were reported as the same in both groups, despite different feeding durations. Trabelsi et al. (2013) demonstrated that during Ramadan, 16 Turkish bodybuilders experienced no difference in body mass or body compositions between the fed and fasted states. Overall, these studies conclude that intermittent fasting either has no influence or a positive influence on body composition; however, none of them noted improvements in performance. This is important for athletes to understand because a leaner body composition does not always equate to improved performance. With other variables aside, a well-fueled athlete will perform better than a leaner, underfueled athlete.


In summary, studies on athletic performance and intermittent fasting are varied and results have been inconclusive. While benefits such as weight loss and improved body composition and inflammatory markers have been reported, no studies have shown an improvement in any type of athletic performance. It is also important to understand that elite athletes require a lot of calories that need to be spread out throughout the day to support multiple training sessions, which could ultimately make intermittent fasting quite difficult. A registered dietitian is an important member of the sports medicine staff that an athlete should meet with prior to participating in intermittent fasting. The dietitian will look at the initial health of the athlete, their training patterns, and the environment in which the fasting would take place. Additionally, the dietitian can dive deeper into the nature and duration of the fast, which includes the degree of food deficit, the timing of intake, and fluid restriction. More long-term research needs to be conducted on athletes in order to evaluate specific fasting methods and their influence on sports performance of varying intensity and duration. If you know an athlete who is participating in intermittent fasting or interested in it, it is important to refer them to a registered dietitian to determine best practices for that individual.


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